Transient global ischemia in rodents (4 vessel occlusion, 4VO) and humans induces death of hippocampal CA1 neurons and is a model for delayed (non-necrotic) cell death after ischemic brain injury. Events that occur long before neuronal death include caspase activation, cleavage of anti-death Bcl-2 family proteins and large mitochondrial channel activity. We have found that the small molecule Bcl-xL inhibitor ABT-737, which enhances death of tumor cells by inhibition of Bcl-xL, paradoxically protects against neuronal death after 4VO in rats. Bcl-xL is prominently expressed in adult neurons and can be cleaved by caspases to generate a pro- death fragment, DeltaN-Bcl-xL that forms in the hippocampus after global ischemia. We find that ABT-737 administered before or after ischemia inhibits mitochondrial membrane channel activity and downstream cell death. Knock-in mice expressing a caspase-resistant form of Bcl-xL exhibit markedly reduced mitochondrial channel activity and reduced vulnerability to ischemia-induced neuronal death, establishing a causal role for DeltaN-Bcl-xL. We have recently described that, in addition to its role in the outer mitochondrial membrane, full length Bcl-xL also affects inner membrane processes by interacting directly with the F1FO ATP synthase, enhancing metabolic efficiency, increasing the rate of enzymatic activity and increasing inner membrane coupling. We further determined that the site of uncoupling is a leak channel made up of the c-subunit ring of the ATP synthase. We now suggest that the c-subunit leak channel forms the inner membrane component of the calcium-sensitive mitochondrial permeability transition pore (mPTP) and in this proposal we plan to test if, during delayed neuronal death after global ischemia, pro-apoptotic DeltaN-Bcl-xL binds to the F1FO ATP synthase causing uncoupling. In an in vitro model of delayed cell death caused by glutamate toxicity, we find that although high dose ABT-737 enhances cell death and worsens metabolic compromise, in contrast low concentrations of ABT-737 prevent such injurious changes, suggesting that low dose ABT-737 is sufficient for sequestration of DeltaN-Bcl-xL but not full length Bcl-xL. We will further test this hypothesis with an aim to determine how the interaction between DeltaN-Bcl-xL and the F1FO ATP synthase regulates ischemic cell death. We will attempt to define this interaction as a novel therapeutic target in ischemic brain injury.